Database
db={"coll1":[{"a_Id":"abc","name":"xyz","age":5}],"coll2":[{"b_id":"abc","FeeAmount":800000,"invoiceNumber":"A10","Date":"2021-10-29T00:00:00.000+04:00","PaidAmount":200000},{"b_id":"abc","FeeAmount":90,"invoiceNumber":"A20","Date":"2021-10-29T00:00:00.000+04:00","PaidAmount":20}]}
Query
db.coll1.aggregate([{$lookup:{localField:"a_Id",from:"coll2",foreignField:"b_id",as:"data",}},{$unwind:"$data"},{$replaceRoot:{"newRoot":{"$mergeObjects":["$$ROOT","$data"]}}},{$project:{"data":0}}])